1. Field of the Invention
This invention relates to a magnetic circuit member. More particularly, this invention relates to a magnetic circuit member that can be employed in a magnetic circuit in a solenoid valve or in a magnetic sensor including a core, a yoke and a housing.
2. Discussion of the Background
A material for forming a magnetic member in a solenoid employed in a pressure control valve is disclosed in Japanese Patent Application Publication Toku-Kai-Hei 10 (1998)-171539, published Dec. 12, 1998. In this conventional pressure control valve, pure iron with soft magnetism or low-carbon steel such as S10C is used to form a magnetic circuit. When electric current is fed to the solenoid coil and the solenoid coil is magnetically excited, magnetic flux is generated by the solenoid coil. The magnetic flux passes through a yoke portion and a plunger portion of the pressure control valve to move and displace the plunger portion. A sleeve connected to the plunger moves with the plunger to control oil pressure.
Recently, it has become desirable for the solenoid used for controlling the oil amount or the oil pressure in a pressure control valve to have a good response to an externally applied magnetic field and to exhibit linearity in magnetic attracting force in order to improve the controllability of the solenoid. If the coil of the solenoid has a good response, the solenoid can be always slightly movable, balancing forces applied to the plunger in order to prevent the plunger portion from being locked.
If the magnetic attracting forces generated by the solenoid coil can be increased linearly relative to an electric voltage applied to the solenoid coil, a control circuit for controlling the solenoid coil can be simplified. As a result the control circuit can be manufactured at low cost. However, when pure iron or low-carbon steel is used to form the magnetic circuit member of the solenoid, the desired response and linearity of magnetic attracting force cannot be achieved.
Conventional magnetic members made from pure iron or low-carbon steel are manufactured by cutting and machining bar-shaped samples of pure iron or low-carbon steel. However, cutting and machining are expensive processes. Furthermore, if a magnetic member needs to be formed into a complex shape, then a large amount of material tends to be wasted in the machining process. This further increases the manufacturing cost of a magnetic member of a solenoid.
An inexpensively produced magnetic circuit member for a solenoid is needed that has a good response to an external magnetic field and linearity in magnetic attracting force relative to electric voltage applied to the coil of the solenoid.
The present invention provides a magnetic circuit member that can be manufactured at low cost, that has a good magnetic response, and that has good magnetic linearity to a magnetic field applied from the outside of the magnetic circuit member.
Research by the present inventors has demonstrated that the deterioration of the magnetic properties of conventional magnetic circuit members (yoke portion, plunger portion and so on) is generated by an alternating magnetic field.
Typically, the solenoid coil and the like are applied with an alternating electric current at 100 Hz to 300 Hz, and then the plunger portion is slightly vibrated. But, in the conventional art, when an alternating electric current flows in a magnetic circuit member made of pure iron or low-carbon steel, eddy currents flow in the magnetic circuit member. The permeability of the magnetic circuit member when eddy currents flow is smaller the permeability of the magnetic circuit member when a direct current flows in the magnetic circuit member, leading to a decrease in magnetic flux density. It becomes difficult to pass magnetic flux through the yoke portion and the plunger portion. Accordingly, if the magnetic flux is concentrated in an end portion of the yoke portion when the plunger portion is departed from the yoke portion, then early saturation of the magnetic flux is generated in the end portion of the yoke portion, the flow of the magnetic flux is reduced, and then the response and the linearity are spoiled. The present inventors arrived at the conclusion that improving the permeability of the magnetic flux passing through the material accelerates the improvements in the magnetic property.
According to a first aspect of the present invention, a magnetic circuit member comprises a ferrite matrix including iron and silicon; and graphite particles in the ferrite matrix, where each of the graphite particles has either a spherical shape or a compact vermicular shape. The inclusion of the graphite, which has a relatively large electrical resistance, in the matrix, which has good magnetic properties, prevents eddy currents from forming in the magnetic circuit member and preserves the good magnetic properties of the magnetic circuit member. As a result, magnetic properties of the magnetic circuit member in an alternating magnetic field can be improved. The graphite particles have either a spherical or a compact vermicular (CV) shape so as to not intercept the magnetic flux passing through the magnetic circuit member. Including the graphite in the magnetic circuit member improves liquidity of the material during casting and facilitates casting.
According to a second aspect of the present invention, to improve magnetic properties in the magnetic circuit member the mass of carbon is preferably less than or equal to 3% relative to the entire mass of the magnetic circuit member, and the mass of silicon is preferably more than or equal to 3% relative to the entire mass of the magnetic circuit member. If the mass of the carbon is larger than 3%, then the magnetic flux density of the magnetic circuit member and the magnetic attracting force are too small and the magnetic circuit member cannot achieve a desired performance.
According to a third aspect of the present invention, the matrix portion preferably includes an iron-silicon compound.
According to a fourth aspect of the present invention, the mass of carbon in the matrix is 0.03% or less relative to the mass of the matrix. If the mass of carbon is larger than 0.03%, then the magnetic property (especially soft magnetism) is substantially spoiled. In particular, the maximum permeability xcexcm tends to be smaller, and the coercive force tends to be larger.
According to a fifth aspect of the present invention, an average particle diameter of the graphite is preferably less than or equal to 50 xcexcm in order to prevent the graphite from intercepting the magnetic flux passing through the magnetic circuit member.